VLBA observations of GHz-Peaked-Spectrum radio sources at 15 GHz

¹ Istituto di Radioastronomia del CNR, CP 141, 96017 Noto SR, Italy
² Visitor at the Space Telescope Science Institute, Baltimore, USA
³ Dipartimento di Astronomia, Università degli Studi, via Ranzani 1, 40127 Bologna, Italy
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
⁵ Dipartimento di Fisica, Università degli Studi, via Irnerio 46, 40126 Bologna, Italy
⁶ Istituto di Radioastronomia del CNR, Via Gobetti 101, 40129 Bologna, Italy

Corresponding author: C. Stanghellini, carlo@ira.noto.cnr.it

Received: 7 June 2001
Accepted: 2 August 2001

Abstract

We present VLBA observations at 15 GHz of ten GHz Peaked Spectrum (GPS) radio sources. The cores are often difficult or impossible to locate. When likely cores are found, they account for a small fraction of the flux density in GPS galaxies -around or below 2% , while in GPS quasars they can account for more than 20% of the total flux density. We detect low polarization in the GPS sources - i.e., typically less than a few percent and often less than one percent. This establishes that low polarization in the parsec scale structure is an important defining characteristic of the GPS sources. The dichotomy in the radio morphology versus optical identification, i.e., galaxies are symmetric and quasars are not, is basically confirmed from these new data, which also indicate that the radio emission from GPS quasars is dominated by a jet, with often a weak or hidden core, suggesting they are at moderate angles to the line of sight, and so are only moderately beamed.